Methods of forming color printings, color duplicates or color proofs (proofs for printing) which comprises conducting development with electrophotographic developing agents to form a plurality of overlapping color toner images directly on the surface of electrophotographic light-sensitive element and transferring at once the resulting color images onto a receiving material such as printing paper are known.
The developing methods include a so-called dry type developing method and wet type developing method. Color images obtained by the wet type developing method are preferred because of little color shear and good resolution as compared with those formed with dry toners. However, it is very difficult to directly transfer wet type toner images entirely from the surface of the light-sensitive element to printing paper.
In order to solve this problem, a transfer technique in which a non-aqueous solvent is supplied between a light-sensitive element and a receiving material and then transfer is electrostatically performed is described in JP-A-2-272469 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Also, a method in which a transparent film is first laminated on the surface of a light-sensitive element, wet type toner images are formed by an electrophotographic process on the film, and then the film bearing the toner images is separated from the light-sensitive element and stuck on paper, thereby forming transferred images is described in JP-A-2-115865 and JP-A-2-115866. According to the method, the film to be laminated has suitably a thickness of 9 .mu.m. However, the production and handling of a film having such thickness is very troublesome and it is necessary to arrange a special system for them.
Further, in JP-B-2-43185 (the term "JP-B" as used herein means an "examined Japanese patent publication"), a method in which imagewise exposure through a transparent electrophotographic light-sensitive element and development are conducted repeatedly to form overlapping color separation images on a dielectric support releasably provided on the light-sensitive element and the dielectric support bearing the images is transferred to a receiving material is described. Since the imagewise exposure is performed from the side of substrate for the light-sensitive element according to this method, the substrate is required to be transparent. This is disadvantageous in view of a cost.
On the other hand, an electrophotographic transfer method using a so-called dry type developing method in which a releasable transfer layer is provided on the surface of a light-sensitive element, toner images are formed on the transfer layer and the toner images are transferred together with the transfer layer to printing paper is described in JP-A-1-112264, JP-A-1-281464 and JP-A-3-11347.
However, in order to obtain good color images by a color image-forming method in which toner images are transferred together with the transfer layer to a printing paper various kinds of requirements must be satisfied.
First, it is important that the transfer layer should be uniform in order to perform uniform charging and exposure to light and not degrade electrophotographic characteristics (e.g., chargeability, dark charge retention rate and photosensitivity) of an electrophotographic light-sensitive element since toner images are formed upon an electrophotographic process. Also, the transfer layer is desired to have good releasability from an electrophotographic light-sensitive element and good adhesion to a receiving material in order to conduct easy transfer in the transfer step. Particularly, an enlarged latitude of transfer conditions (for example, heating temperature, pressure and transportation speed) is required. Moreover, it is desired that a color duplicate obtained accept retouching and sealing without causing any trouble and have good storage stability, for example, in that the transfer layer is not peeled off when the color duplicate has been filed between plastic sheets and piled up.
However, these characteristics have not been fully considered in the techniques hitherto known and image forming performance of color image, transferability of transfer layer and retouching property, sealing property and storage stability of color duplicate are not satisfactorily good.
Also, in order to employ the light-sensitive element repeatedly in the techniques hitherto known, a special operation is required at the time of transfer or difficulties in the formation of transfer layer are encountered. On the other hand, the method using a light-sensitive element having a transfer layer (or a releasable layer) previously formed thereon is disadvantageous in point of cost since the light-sensitive element used is inevitably thrown.